(Color online) Two consecutive gray-scale EBL steps are applied to a single PMMA layer. The resist is selectively transformed from a stepped structure into a sloped sidewall based on a molecular weight dependent thermal reflow.
Contrast curves of 1 μm thick 950 k PMMA resist electron beam exposed at 100 keV/1 nA and subsequently developed in undiluted MIBK at 20 °C for 30 s (standard) and 900 s, respectively.
Molecular weight of 950 k PMMA resist exposed to different doses analyzed by gel permeation chromatography. (a) Two groups of the molecular weight distributions are plotted which correlate to a high and a low dose gray-scale EBL. (b) A graphical evaluation of the number average molecular weight Mn over the applied dose shows a decay of the molecular weight for increasing dose values.
Summary of a reflow survey: a 1 μm high five-level PMMA pattern was exposed to different temperatures for various durations. Vertical features remain steeper if 110 °C is applied for a considerably increased time of 120 min (scale bar: 2 μm).
Sloped and stepped resist profiles in 1 μm PMMA is accomplished by combining two consecutive gray-scale exposures based on different contrast curves (a) and a thermal postexposure treatment (b). Only a high dose gray-scale exposure to PMMA leads to a Mn reduction below the Tg -relevant value of 10 k, which allows the selective melting due to a significant but locally defined shift in the Tg . For the two examples shown, the (unexposed) center levels unintentionally have different widths due to a misalignment during EBL exposure (scale bar: 2 μm).
Quantitative analysis of the number average Mn and the weight average Mw of the PMMA resist as function of the exposure dose.
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